Direct and indirect dissociative double photoionization of diatomic molecules

Abstract

An enhanced photoelectron - ion - ion triple coincidence technique eliminates the effect of target molecule thermal velocity on measured ion pair kinetic energy releases. Kinetic energy release distributions at photon energies of 40.8 and 48.4 eV are reported for diatomic molecules and are compared with calculated distributions for direct vertical transitions to theoretically known dication states. Good agreement in the case of indicates that double photoionization of this molecule at 8 eV above threshold is mainly direct. Double photoionization of CO, and NO is purely indirect at the lowest photon energies and has both indirect and direct components at higher energies in this range. The indirect processes give a wide range of final product energies and often involve atomic autoionization. Selectivity in the direct double ionization processes is consistent with the application of symmetry-based propensity rules.